1. Field of the Invention
The present invention relates to an improved battery charging technique for rechargeable spacecraft nickel/hydrogen (Ni/H.sub.2) batteries.
2. Discussion of the Prior Art Battery charge and discharge rates are often expressed in a C/X notation (e.g. C/2). This notation, which will be employed throughout this disclosure, normalizes the rates so that they are independent of battery capacity or battery size. In the numerator, battery capacity C is usually expressed in Ah (Ampere-hours), which is a quantity of charge. The denominator X represents the number of hours required to either charge or discharge the full battery capacity. Therefore, C/x has units of Amperes, which is a charge or discharge rate. For example, a C/22 discharge rate means that full battery capacity will be discharged in two hours. If a particular battery is rated at 100 Ah capacity, then a C/22 discharge rate would be 50 Amperes and the battery would be fully discharged in two hours.
Another term commonly used in this disclosure is "recharge ratio". The recharge ratio may be defined as the ratio of the quantity of charge, measured in ampere hours, imparted to the battery to the quantity of charge, also measured in ampere hours, available for discharge by the battery.
Commonly assigned U.S. Pat. No. 5,395,706 to Hall, the entire disclosure of which is hereby incorporated into the instant disclosure by reference, describes a unique method of operating a nickel-hydrogen battery when the battery is less than fully charged and results in increasing the charge capacity of the battery. The method comprises the step of completing the recharging process for the battery at a temperature T.sub.1 in the range of approximately -10.degree. C. down to -30.degree. C. which is lower than a temperature T.sub.2 in the range of approximately -10.degree. C. to +50.degree. C. at which discharge begins As related in the patent, it is desirable, subsequently, to heat the battery to the temperature T.sub.2 in readiness for discharge. A preferred recharge temperature is less than approximately -10.degree. C.
Ni/H.sub.2 batteries will charge to a higher state-of-charge and will store more electrical capacity if they charge with high current or power. However, higher charging currents also cause an increase in the rate of a parasitic electrochemical reaction that evolves oxygen gas (near end-of-charge), which generates heat, wastes power and energy, and shortens the charge/discharge life of the battery. Today's geosynchronous earth orbit (GEO) spacecraft Ni/H.sub.2 batteries may undergo charge/discharge cycling for 15 or more years. This requirement dictates low charging currents to achieve battery life requirements at the expense of capacity.
Some LEO (low earth orbit) satellite applications require both rapid charge rates and long battery cycle life. To minimize the adverse effects of the high charge rates on cycle life, LEO spacecraft batteries sometimes charge at a high rate initially while deleterious gas evolution reaction rates are low, and then the charge current steps down or tapers down to a lower value during overcharging when gassing rates increase. After charging and properly overcharging, Ni/H.sub.2 and NiCd batteries are often trickle charged at very low rates until discharging begins. This charging method is baselined for the Space Station batteries. Similar charging techniques apply to lead/acid and other batteries in terrestrial applications, and commercial battery charge controllers automatically reduce charge current at a preset charging voltage. However, reducing charge current near end-of-charge does not optimize battery capacity. Also, initial high rate charging followed by taper charging is not used commonly, if ever, for GEO spacecraft battery charge profiles.
As described in the Hall patent, the capacity of Ni/H.sub.2 batteries can also be increased substantially by recharging at a significantly lower temperature than the discharging temperature. To implement this technology practically on a satellite, the heat removal radiator size must be increased in order to cool the battery to its optimum recharge temperature. However, even with a larger radiator, optimum recharge temperatures cannot be practically achieved using conventional constant current or constant power recharge methods. Battery heating, especially during overcharge, will raise temperatures beyond optimum for maximum capacity storage.
The present invention is an improvement on the Hall patent. While utilizing the basic teachings of the Hall technique, the present invention provides more specific operating steps which further advance that technique, making it even more valuable.